Hi Charles. Here is some info on my solution since it was last raised.
In 2008 my old spark (756) was quite loose in the beam sockets and I built a wooden 'table' on which I could sit the boat supported by the beams with the hulls hanging either side (trampoline on and high enough to fit the rudders fully down). This enabled me to do some measuring of hull movement, hull alignment and rudder alignment - and showed that my hulls were 20mm closer at the bow than the stern, using the skeg line width as the measuring point.
The problem resulted from wear in the beam sockets leading to the front beam stops (the white brackets under the beams) wearing the hull surface. Whilst I could have easily built up the hull and replace the beam stops this would not have cured the grinding problem. I too looked at gelcoating in the sockets but it was not going to be easy to build up evenly.
In the end I did some experimenting with duck tape to see how much thickness I needed to be able to insert and withdraw the beams but remove the 'slop'. I had some reinforced reservoir liner left over from shed roofing and I found that by using one thickness of that over the whole socket width of the beam - glued on with evostik worked perfectly on both the front and rear beams and left the boat much tighter. In order to help prevent the hull wear at the beam stops I chose not to build up the hull or replace the brackets. Instead, again by measurement, I found that a 3mm thick rectangle of hard neoprene glued to each of the front beam brackets was sufficient to align the hulls. I believed that this semi flexible material would remove some of the shock loading in heavy seas and would prevent wear.
I then also did similar work on our Dart 18 and can report that, following two seasons of sailing in everything up to the top of F6 the mods have worked perfectly (and the beams can still be removed). If anyone is interested in trying this out I used Fawcett Liners No.10 reservoir liner (woven thread in UV protected PVC) from
http://www.fawcettsliners.co.uk
In terms of longevity of this material, the first time I used it was on my garage and outbuildings roof in about 1990 and that sheet is still there 20 odd years later with no discernable deterioration. It is sold primarily for water board and farm reservoirs. £3.84 a sq metre at present.
If anyone coming to the SSC Training weekend in April - my boat table is in the SSC field and I will level it up beforehand so it can be used to check boat alignment for anyone interested.
By the way - having realigned the hulls using the boat table I then found that the rudders were not vertical or aligned with the skeg line. I found that the top hull fixing for one of the rudders was not in its right position. It was tight but when I undid it I found that the bolt holes had worn in the hull and had allowed the fitting to be moved about 2-3mm outwards - giving about 15mm of misalignment at the tip of the blade. I reglassed the holes and then redrilled to refix the fitting in the correct position.
